Series motors, and especially universal motors, suitable for selective connection to a-c and d-c power networks, are frequently used to form the mechanical energy sources for operator-handled appliances. Typical applications are electric hand tools, hedge shears, chain-saws, lawn-mowers, grinders, planers, or, in general, hand-operated or personally used machinery which includes a cutter. Upon disconnection of the motor from a power network, the kinetic energy of the drive causes the motor to continue to rotate and, hence, cutter blades and the like also continue to operate. The continuously operating cutter blades, shears, saw blades or chains, grinding disks or other apparatus can be a hazard to the operator. When disconnecting the tool, the motor and hence the cutter blade, must be stopped quickly. Mechanical brakes to stop the tools are not suitable, since they are expensive to make, require continuous maintenance, and are subject to malfunction unless properly serviced. Usually, circuits are coupled to the motor in which the motor, after disconnection from the supply network, is changed over to generator operation. The energy generated during generator operation is applied to a load resistor which dissipates the energy in form of heat which is radiated to ambient space.
German Patent Disclosure Document DE-OS 30 35 185 describes an arrangement in which the braking or dissipating current is connected to the armature circuit of a series motor under control of a negative temperature coefficient (NTC) resistor. A braking current or dissipating current will flow through the armature network until the motor, with the tool, cutter, or other appliance connected thereto, is at least approximately stationary.
To operate a universal motor as a generator or, in other words to selectively operate a dynamoelectric machine as a motor or a generator, it is necessary that the field winding supply some magnetic field so that the machine can be self-excited. Self-excitation of the field thus must be derived from the remanence of the field structure itself. Such remanence is sometimes insufficient in order to initiate self-excitation. This is particularly so if the motors have somewhat higher power rating. Thus, reliable generator operation of the ill rotating although disconnected motor and the attached appliance or device could not be ensured.